Sex differences in wrist strength: a systematic review

Sex differences in strength have been attributed to differences in body anthropometrics and composition; these factors are often ignored when generating workplace guidelines. These differences directly impact the upper extremity, leaving female workers exposed to injury risk. The wide range of tools and techniques for measuring upper extremity strength presents a challenge to ergonomists and work task designers; collating outcomes to provide a clear outlook of differences between males and females is essential and the purpose of this work. Four online databases were searched (PROSPERO ID: CRD42022339023) with a focus on articles assessing sex differences in wrist strength. A total of 2,378 articles were screened for relevancy; 25 full-text articles were included in this systematic review. Articles examined movement pairs (ulnar/radial deviation, pronation/supination, and flexion/extension), as well as contraction types (isometric and isokinetic) to observe sex differences in wrist strength. Across all articles, females produced ∼60–65% of male flexion/extension strength, ∼55–60% pronation/supination strength, and ∼60–70% ulnar/radial deviation strength. Overall, females presented lower strength-producing abilities than males, but when considering strength relative to body mass, male-female differences were less pronounced and occasionally females surpassed male strength metrics; typically, this occurred during flexion/extension, particularly in isokinetic contractions. This review has identified a scarcity of articles examining ulnar/radial deviation, pronation/supination, as well as isokinetic contractions; these are needed to supplement workplace exposure guidelines.


INTRODUCTION
Sex differences in strength generation are multifactorial.In the context of human capacity, strength is the ability to generate maximal force.Sex differences in strength have been attributed to differences in body composition and anthropometrics (Glenmark et al., 2004).At the same body mass index (BMI), males tend to have more lean muscle mass while females have more body fat than males (Bredella, 2017;Tyagi et al., 2017).These sex differences are more apparent during puberty when the production of sex hormones is increased (Bredella, 2017;Tyagi et al., 2017).These differences are more visible in the upper extremity; in absolute values and relative to body mass males have significantly greater skeletal muscle mass than females (Miller et al., 1993;Janssen et al., 2000).When examining equivalently trained athletes, females typically produce 40-75% of male absolute strength capabilities (Bartolomei et al., 2021).The strength-anthropometric relationship supports the notion that greater anthropometric measurements such as height, weight, limb length, and cross-sectional area (CSA) lead to greater strength production.This is supported by a larger CSA composed of a greater number of motor units and muscle fibers, leading to an increase in the ability to produce force (Bartolomei et al., 2021;Qazi et al., 2017).This potential mismatch in strength capability can affect activities of daily living, sports settings, or in the workplace.
The prevalence of female workers in the global labour force has increased.In 2014, 47% of the US labour force were women, and these projections suggest that by the year 2050, the number of female workers in the United States will surpass 92 million (Cruz Rios, Chong & Grau, 2017).Despite the increase in female workers, there is a lack of understanding of the differences in workplace risks between sexes (Tessier-Sherman et al., 2014).In jobs that require physical strength, female workers are more susceptible to workplace musculoskeletal disorders (WMSDs) as job demands represent a higher proportion of physical capacity (Tessier-Sherman et al., 2014;Blue, 1993).As job demands are typically indiscriminate of the sex of the worker, females are left with a higher risk of WMSDs than male counterparts in typical blue-collar jobs (Taiwo et al., 2008).Females are more susceptible to upper extremity repetitive strain injuries (RSIs) due to their smaller stature, hormonal differences, and decreased strength production (Taiwo et al., 2008).This correlates to an increased risk for carpal tunnel syndrome (CTS) and tendinopathies due to repetitive strain and physical demand requirements, particularly in females (Fan et al., 2009;McDiarmid et al., 2000;Shiri & Viikari-Juntura, 2011;Wolf et al., 2010).
To prevent this increased risk of WMSDs in female workers, understanding differences in strength capabilities between sexes is essential.While differences in wrist strengths between sexes have been documented, due to the large variabilities in strength findings across literature, task design and ergonomic guidelines are often driven based on a singular or small number of former research studies, which may have dramatic effects on what is deemed ''acceptable'' or ''unacceptable'' for workplace strength requirements.Workplace tasks may not scale force or loading requirements to worker anthropometrics or capability, placing disproportionate risk levels on those with decreased capacity.As these differences often align with sex differences, incorporating this into industrial design would provide the ability to strengthen workplace guidelines regarding the distal upper extremity to maximize worker safety.There is a wide body of research examining sex differences and wrist strength, but interpretation is hampered by varied measurement techniques and participant populations; collation of this existing research is essential to allow informed decision making by researchers.The purpose of this review was to assess the current literature on sex differences in wrist strength and to combine these findings to quantify changes in strength-generating abilities.

MATERIALS AND METHODS
A search was conducted considering the main topics and selecting a concise list of keywords used to effectively extract relevant articles for consideration.Four databases were searched  1).
The search string was used to ensure relevant articles providing information regarding the forearm and wrist segment and strength outcomes for both sexes were discovered.The study was registered to PROSPERO (ID: CRD42022339023).
Articles were deemed eligible if they contained the desired study design that focused on sex differences in strength metrics at the wrist.Studies were required to include both sexes and were limited to healthy individuals between 18 and 65 with no prior acute pathologies of the upper extremity, or chronic pathologies such as cancer or post-stroke diagnoses that could have impacted muscular strength.Articles needed to include strength modalities, with outcome measures for participant populations split between sexes.Exclusion criteria included incorrect study design, results not split by sex, wrong outcome measures, or if the article was not written in English.
A multi-step screening process was completed to filter articles for final data extraction.Initial database searches were performed based on inclusion/exclusion criteria.Articles were uploaded into Covidence (Veritas Health Innovation Ltd., Melbourne, Australia) to organize the screening processes.Titles and abstracts were examined first by two independent reviewers, with a third reviewer to resolve conflicts.Following initial screening, the same inclusion/exclusion criteria and conflict resolution approach was used in full-text review for eligible articles (Fig. 1).All articles that were accepted after the second round of screening were included for data extraction.
During extraction, all included articles were assessed for potential risk of bias using the Risk of Bias in Non-randomized Studies (ROBINS-I).Seven domains used to determine the level of bias within each article, including bias due to confounding, selection of participants, clarification of interventions, deviations from intended interventions, missing data, measurement outcomes, and selection of the reported results.Ratings were described as low (L), moderate (M), and serious (S) and a total bias rating was generated based on the highest score across domains (Table 2).
Following screening, relevant outcome information was extracted.Principal summary measures extracted from all articles were recorded strength metrics for males and females in each experimental task.In addition, participant demographics, measurement method, movement direction and type were extracted.Where available, effect sizes of these strength Full-size DOI: 10.7717/peerj.16557/fig- 1 differences were recorded or calculated by using means and standard deviations between male and female groups and was included within the extraction tables.

Article selection
Collectively, 3,610 articles were collected through the initial database search; after the first screening stage and removal of duplicates, 2,306 articles were deemed irrelevant and 72 were full-text assessed for eligibility.Of these 72 articles, 47 were removed due to exclusion criteria, leaving 25 articles for extraction.A detailed flowchart of article screening is detailed in Fig. 1.

Strength measurements
Wide ranges of strength outcomes were observed but collectively demonstrated decreased female strength relative to males; when normalized to male strength, variability existed between movement directions.These have been separated by movement planes below.
Sex differences in pronation/supination strength were documented to have less variability across articles when compared to other movement patterns, with females generating ∼55-60% of male strength (Tables 7 and 8).Extracted female strengths ranged from 46-98%  and 45-95% of male strengths in pronation and supination, respectively (Axelsson et al., 2018;Harbin, Leyh & Harbin, 2020;Kramer et al., 1993;Matsuoka et al., 2006;Nilsson et al., 2019;Puharic & Bohannon, 1993) (Fig. 4).The greatest percentage disparity in pronation strength occurred when females generated isometric torques of 3.33 Nm when in complete pronation and 5.10 Nm while pronating in a supinated posture, where males produced 7.24 Nm and 10.12 Nm, respectively (Matsuoka et al., 2006).Substantial supination differences were observed during neutral isometric contractions, where females 18-35 yrs produced torques of 4.1 Nm and 3.8 Nm at 36-65 yrs, where males generated 6.1 Nm and 8.4 Nm, respectively (Nilsson et al., 2019).Minimal sex differences were observed in non-dominant hand contractions, where females were able to generate 94 N of isometric force compared to 96 N in males (Axelsson et al., 2018); this was the only article that displayed such similar values.Similar values between sexes were only observed once in isokinetic trials at 60 • /s and 120 • /s, where females produced 9.7 Nm and 9.3 Nm, where males generated 10.2 Nm and 9.8 Nm, respectively (Kramer et al., 1993).

DISCUSSION
The primary aim of this review was to examine sex differences in wrist strength during different movement directions and contraction types.Although sex differences in strength have been heavily investigated, there is a paucity in research regarding sex differences in strength of the distal upper extremity.From this review, sex differences in wrist strength were observed across articles, with females commonly producing lower strength metrics than males with very few exceptions.These differences ranged based on different movement patterns and contraction types, with females generating considerably smaller indexes.Importantly, this review provides ranges, noting that, across all articles, females typically generated ∼60-65% of male flexion/extension, ∼55-60% of pronation/supination, and ∼60-70% ulnar/radial deviation strength.Ergonomists and work task developers should account for decreases in relative strengths that differ based on movement direction and contraction types to improve occupational workplace guidelines.Female strength centralized around ∼55-70% of male strength, but differences existed across movement pairs.Females produced a wider percentage of strength during flexion/extension with a range of 38-144% of male strength; pronation/supination and ulnar/radial deviation only presented ranges of 45-98% and 48-80%, respectively.This is important to note as females generated reduced relative strength during pronation/supination movements, and more so during ulnar/radial deviation.In activities of daily living, individuals commonly adopt the dart-throwers motion (DTM), which is described as moving from extension and slight radial deviation to flexion and slight ulnar deviation (Anderton et al., 2022).Many hand tool tasks place workers under high degrees of ulnar/radial deviation, leaving workers with a greater risk of injury during these movements due to decreased strength ability (Rempel, Keir & Bach, 2008).These differences in strength across movements are relevant for occupational ergonomists and work task designers, who can leverage these strength ranges for industrial task design; particularly in considerations of increased conservatism for strength requirements when the task requires wrist movement directions outside of flexion/extension.Isometric exertions were the predominant measurement method for assessing strength in these included articles, where females produced less strength when compared to isokinetic contractions.Females generated ∼38-82% of male flexion strength during isometric contractions, and 53-135% during isokinetic.When examining extension movements, females were able to generate ∼42-79% and ∼49-102% of male strength during isometric and isokinetic contractions, respectively.The large ranges in strength capabilities present the need for conservatism in generating work demands; tasks scaling to male strength will need to consider the range of lower female strength percentages presented in this review.These differences tended to become larger when accounting for bodyweight, where during isokinetic contractions, females produced 144% of male strength in flexion/extension movements (Mao et al., 2000).It is possible that normalizing to body mass mitigated the sex effects commonly seen between males and females; additional research under body mass normalized conditions are warranted.Isokinetic contractions and relative strength values were only examined in three articles; this number is insufficient to conclude isokinetic strength generated significant relative strength increases in females, but indicates that further analysis should be conducted on these findings.
This review has identified a large research gap examining sex differences in wrist strength, specifically regarding movement directions, contraction types, and normalized strength metrics.Of the 14 articles examining flexion and extension, only six (Danneskiold-Samsøe et al., 2009;Forthomme et al., 2002;Harbo, Brincks & Andersen, 2012;Hill et al., 2018;Mao et al., 2000;Nicholas et al., 1989) examined isokinetic contractions; four articles examined ulnar and radial deviation, but none completed isokinetic testing, and only one article was found to examine isokinetic contractions during pronation/supination (Kramer et al., 1993).Isokinetic contractions are currently underrepresented in this research area; expansion of findings would provide improved insight into sex differences in wrist strength.This review has additionally identified relative strength as a large research gap.Relative strength was only examined during flexion/extension and during isokinetic contractions (Forthomme et al., 2002;Mao et al., 2000;Nicholas et al., 1989), identifying a large existing research gap investigating strength and isometric contractions, as well as ulnar/radial deviation and pronation/supination.At present, it is difficult to conclude whether these strength differences are due to anthropometric and muscular composition differences (Miller et al., 1993;Janssen et al., 2000;Bartolomei et al., 2021) or biases that exist in these articles, but a general trend that females produce lesser strength compared to males is clear.
There are limitations to be considered for this review.Substantial variability in study design was observed in the included articles and prevented statistical interpretation via a meta-analysis.The wide array of relative strength percentages outlined in the results may have been generated from confounding outcomes.A minimal number of articles normalized strength metrics to bodyweight, but when this occurred, results conflicted within this review, hampering interpretability (Forthomme et al., 2002;Mao et al., 2000;Nicholas et al., 1989).Further investigations regarding ulnar/radial deviation, pronation/supination and isokinetic contractions are needed to fully understand the complexity of sex differences in wrist strength.Strength differences in injured populations were not examined within this review, where results may differ when including injured populations where sex differences in wrist strength may become greater or lesser with injury.While additional strength research for the wrist existed, many articles were excluded as they exclusively utilized males or females, preventing opportunities for comparison.Many of the studies within this systematic review have small sample sizes, where additional research with larger participant pools would strengthen our understanding of these differences in strength.Many articles that identified greater female strength did so, only when strength was normalized to body mass; it is possible that this technique alters our interpretation.However, with few studies using this methodology and an inability to definitively reintegrate these values to absolute strengths without knowing the relationships between participant masses and outcome strengths, we identify this as a potential limitation.Future research would benefit from additional studies using this technique, as well as documenting individual participant outcomes to allow research groups to redefine the outcomes as absolute strengths if they so choose.

CONCLUSIONS
Female strength was observed to be lower than male strength across all wrist motions during isometric contractions for articles included within this review.Some articles demonstrated higher female strength, but these were typically only observed when strength was normalized to participant body weight.Job demands do not often differ based upon anthropometrics and are generally created with the focus on absolute strength, leaving female workers exposed to injury risk.Female workers are expected to work under the same conditions as male workers, therefore requiring them to work at a higher capacity threshold, which may attribute to their increased WMSD risk observed across a magnitude of different occupations (Tessier-Sherman et al., 2014;Islam et al., 2001).The decreased relative strengths are not at universal levels but differ by movement direction and contraction type; these should be considered by ergonomists and work task designers when examining occupational scenarios.These high demands are associated with wrist/hand pathologies such as carpal tunnel syndrome (CTS) which are already more prevalent in females (Feng et al., 2021), indicating the importance of closing this research gap to develop stricter workplace guidelines to aid in injury risk prevention (Fan et al., 2009;Shiri & Viikari-Juntura, 2011;Wolf et al., 2010).

Figure 1
Figure 1 PRISMA flowchart describing screening process and inclusion of eligible articles for this review.Full-size DOI: 10.7717/peerj.16557/fig-1

Table 1 Search string for all databases.
Search string generated from specific search terms to gather relevant articles regarding forearm and wrist segment and strength outcomes for both sexes.June 2022 including Embase, MedLine, ProQuest, and Web of Science.The search was generated, and a finalized search string for each database was produced (Table Muscle OR joint OR isometric OR isokinetic OR dynamicANDSex identificationStrength OR force OR moment OR torque ((Woman OR women OR female) AND (Man OR men OR male))ORSex OR gender on 13

Table 3 Included articles with flexion outcome measures.
Included articles within this table contained outcome measures for flexion, and includes author information, participant information, measurement methods, type of contraction, outcome measures, and then calculated or included effect sizes.

Table 4 Included articles with extension outcome measures.
Included articles within this table contained outcome measures for extension, and includes author information, participant information, measurement methods, type of contraction, outcome measures, and then calculated or included effect sizes.

Table 5 Included articles with ulnar deviation outcome measures.
Included articles within this table contained outcome measures for ulnar deviation, and includes author information, participant information, measurement methods, type of contraction, outcome measures, and then calculated or included effect sizes.